FIG. 10A shows a schematic plan view showing a field-effect transistor disclosed in Japanese laid-open patent publication No. 2007-115894. Comb-shaped drain 10 and source 15 are arranged to be engaged with each other. The drain 10 includes a plurality of drain ohmic contacts 10A, and a drain coupling portion 10B for mutually coupling first ends (right ends in FIG. 10A) of the plurality of drain ohmic contact contacts 10A. Similarly, the source 15 includes a plurality of source ohmic contacts 15A, and a source coupling portion 15B for mutually coupling first ends (left ends in FIG. 10A) of the plurality of source ohmic contacts 15A.
A plurality of gate fingers 20A are arranged to regions between the drain ohmic contacts 10A and the source ohmic contacts 15A. A gate power supply wiring 20B couples first ends (left ends in FIG. 10A) of the plurality of gate fingers 20A. The gate power supply line 20B intersects with the source 15. Both the gate power supply wiring 20B and the source 15 are insulated at the intersection portion therebetween.
A gate width of the transistor (gate finger length) is increased or the number of gate fingers is increased, thereby outputting high data.
A transistor having a gate finger is formed onto a semiconductor substrate having a crystal default of a micro pipe. Then, at the intersection position between the gate finger and the micro pipe, the gate finger is easily uncoupled. In particular, upon using a single-crystal SiC substrate on which the micro pipe is easily caused, the probability for uncoupling the gate finger is high. When the gate finger is uncoupled, the amount of current per gate width is varied, thereby reducing the yield. Particularly, the number of gate fingers for high output is increased. Then, the probability for uncoupling the gate finger is increased and the yield is dramatically reduced.
FIG. 10B shows a plan view showing a microwave switch element shown in Japanese laid-open patent publication No. 10-284508. The drain 10 having two ohmic contacts is engaged with the source 15 having two ohmic contacts. The gate fingers 20A are arranged one by one between the ohmic contact of the drain 10 and the ohmic contact of the source 15. That is, the total number of gate fingers 20A is three.
A first gate power supply line 20X couples first ends of three gate fingers 20A, and a second gate power supply line 20Y couples second ends of the three gate fingers 20A. The first gate power supply line 20X intersects with the source 15, and the second gate power supply line 20Y intersects with the drain 10. The gate finer 20A, the source 15, the drain 10, the first gate power supply line 20X, and the second gate power supply line 20Y are arranged to be point-symmetrical to each other. The arrangement obtains a microwave switch element having symmetrical switching characteristics.
FIG. 10C shows a planar pattern upon applying the gate finger coupling structure shown in FIG. 10B to the field-effect transistor shown in FIG. 10A. One end (right end in FIG. 10C) of the drain coupling portions 10B in the gate fingers 20A is coupled to the second gate power supply lines 20Y. The gate power supply wiring 20B corresponds to the first gate power supply line 20X. The second gate power supply line 20Y intersects with the drain 10.
With the structure shown in FIG. 10C, even if one gate finger 20A is uncoupled at one portion thereof, a gate voltage is applied to a portion on the drain 10 side, rather than the uncoupling portion, via the second gate power supply line 20Y. Therefore, desired transistor characteristics can be maintained.
In the field-effect transistor shown in FIG. 10C, the gate power supply line 20Y intersects with the drain. Therefore, the number of parasitic capacitance between the gate and the drain is increased and preferable high-frequency characteristics cannot be maintained.